Metal gate and contact plug design and method forming same

ABSTRACT

A method includes forming a dummy gate stack over a semiconductor region, forming a gate spacer on a sidewall of the dummy gate stack, removing the dummy gate stack to form an opening, forming a replacement gate stack in the opening, recessing the replacement gate stack to form a recess, filling the recess with a conductive material, and performing a planarization to remove excess portions of the conductive material over the gate spacer. A remaining portion of the conductive material forms a gate contact plug. A top portion of the gate contact plug is at a same level as a top portion of the first gate spacer.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.15/429,894, filed Feb. 10, 2017, and entitled “Metal Gate and ContactPlug Design and Method Forming Same,” which claims the benefit of thefollowing provisionally filed U.S. Patent application: Application Ser.No. 62/368,505, filed Jul. 29, 2016, and entitled “Metal Gate andContact Plug Design and Method Forming Same,” which applications arehereby incorporated herein by reference.

BACKGROUND

With the sizes of the integrated circuits becoming increasingly smaller,the respective formation processes also become increasingly moredifficult, and problems may occur where conventionally no problems willoccur. For example, in the formation of Fin Field-Effect Transistors(FinFETs), the metal gates and the adjacent source and drain regions maybe electrically shorted to each other. The contact plugs of metal gatesmay also be shorted to the adjacent contact plugs of source and drainregions.

In addition, the formation of FinFETs may involve the formation of dummypolysilicon gates, removing the dummy polysilicon gates in subsequentprocesses, and filling the recesses left by the dummy polysilicon gateswith replacement metal gates. However, since the dummy polysilicon gatesbecome very narrow, polysilicon residue may be left due to theincomplete removal of the dummy polysilicon gates, causing thedegradation of the performance of the devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 23 illustrate the cross-sectional views of intermediatestages in the formation of transistor in accordance with someembodiments.

FIG. 24 illustrates a process flow for forming a transistor inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A transistor with metal gates and contact plugs and the method offorming the same are provided in accordance with various exemplaryembodiments. The intermediate stages of forming the transistor and themetal gates and contact plugs are illustrated in accordance with someembodiments. Some variations of some embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements.

FIGS. 1 through 23 illustrate the cross-sectional views of intermediatestages in the formation of a transistor in accordance with someembodiments of the present disclosure. The steps shown in FIGS. 1through 23 are also reflected schematically in the process flow 500shown in FIG. 24. The illustrative embodiments use the formation of aFin Field-Effect Transistor (FinFET) as an example. The concept of thepresent disclosure is readily applicable to planar transistors.

Referring to FIG. 1, semiconductor substrate 20, which is a part ofsemiconductor wafer 2, is provided. In accordance with some embodimentsof the present disclosure, semiconductor substrate 20 includes acrystalline silicon. Other commonly used materials such as carbon,germanium, gallium, boron, arsenic, nitrogen, indium, phosphorus, and/orthe like, may also be included in semiconductor substrate 20. Substrate20 may also be a compound substrate including a III-V compoundsemiconductor or silicon germanium.

Semiconductor substrate 20 includes portions in a plurality of regionsin which different types of transistors are to be formed. In accordancewith some embodiments, the plurality of regions includes standard deviceregion 100, High-Voltage (HV) device region 200, long-channel deviceregion 300, and Input-Output (IO) device region 400. Standard deviceregion 100 may be used for forming core/logic transistors. HV deviceregion 200 may be used for forming HV transistors. Long-channel deviceregion 300 may be used for forming transistor having long channels, andIO device region 400 may be used for forming IO transistors. The coretransistors may be supplied with a low positive power supply voltage,for example, lower than about 1.0 volt. The HV transistor may besupplied with, and are configured to endure, a high positive powersupply voltage higher than the positive power supply voltage of thedevices in device region 100. For example, the power supply voltages ofthe HV device regions 200 may be between about 1.5V and about 3.3V.

In accordance with some embodiments of the present disclosure, substrate20 includes semiconductor fin(s) 20A, which are the portions overadjacent isolation regions. An exemplary fin 20A is shown in FIG. 5B,which illustrates that semiconductor fin 20A is higher than the adjacentisolation regions 38, which may be Shallow Trench Isolation (STI)regions. It is noted that although for simplicity, the fins in regions100, 200, 300, and 400 are illustrated as connected to each other, inreality, the fins in different device regions are separate from eachother.

Referring back to FIG. 1, dummy gate dielectric 22 is formed oversubstrate 20. Dummy gate dielectric 22 may be formed of silicon oxide.Dummy gate electrode 24 is formed over dummy gate dielectric 22, and maybe formed of polysilicon in accordance with some embodiments.

Mask stacks 112, 212, 312, and 412 are formed in device regions 100,200, 300, and 400, respectively, and have widths W112, W212, W312, andW412, respectively, with the relationship W412>W312>W212>W112 inaccordance with some exemplary embodiments. Mask stack 112 may includelayers 114, 116, and 118, which may be formed of silicon oxide, siliconnitride, silicon carbo-nitride, silicon-carbon-oxynitride, or the like.Mask stacks 212, 312, and 412 have the same layers as mask stack 112.

Bottom Anti-Reflective Coating (BARC) 26 is formed over dummy gateelectrode 24 and fills the space between mask stacks 112, 212, 312, and412. In accordance with some embodiments, BARC 26 is formed of adielectric material such as SiON. BARC 26 may be filled to cover maskstacks 112, 212, 312, and 412, or may have a top surface level with orhigher than the top surface of layer 118. Patterned photo resist 28 isformed over BARC 26, and covers mask stacks 212, 312, and 412, leavingmask stack 112 not covered.

Referring to FIG. 2, BARC 26 is patterned using photo resist 28 as anetching mask. Accordingly, the sidewalls of mask stack 112 are exposed.Next, a trimming step is performed, wherein the trimming is isotropic,and may be a wet etching process or a dry etching process. Therespective step is illustrated as step 502 in the process flow shown inFIG. 24. In accordance with some embodiments, the trimming is performedthrough wet etching process using an etching solution including HF,de-ionized water, NH₄OH, H₂O₂, Isopropanol (IPA). Alternatively, thetrimming is performed through a dry etching process using an etching gasincluding HBr, SO₂, or CF₄. As a result of the trimming, mask stack 112has width W112′, which is smaller than about 0.9×W112 in accordance withsome embodiments.

Next, BARC 26 and photo resist 28 are removed, as shown in FIG. 3.Another trimming process may be performed to further trim the widths ofmask stacks 112, 212, 312, and 412 using isotropic etching. Therespective step is also illustrated as step 502 in the process flowshown in FIG. 24. The etchant may be the same as (or different from) theetchant used in the trimming as shown in FIG. 2. As a result, widthW112″, W212′, W312′, and W412′ are smaller than about 90 percent ofwidths W112′, W212, W312, and W412, respectively, as shown in FIG. 2.

Referring to FIG. 4, dummy gate electrode layer 24 and dummy gatedielectric 22 (FIG. 3) are etched using mask stacks 112, 212, 312, and412 as an etching mask. The remaining portions of the etched dummy gateelectrode layer 24 are dummy gate electrodes 124, 224, 324, and 424. Theremaining portions of the etched dummy gate dielectric 22 are dummy gatedielectrics 122, 222, 322, and 422, respectively. The respective step isillustrated as step 504 in the process flow shown in FIG. 24. Duringthis step, dummy gate dielectric layer 22 (FIG. 3) is etched through,and semiconductor fin 20A is exposed. The sidewalls of dummy gatedielectrics 122, 222, 322, and 422 are also exposed. Mask layers 118 asshown in FIG. 3 may be consumed.

Through the process as shown in FIGS. 1 through 4, the widths of dummygate electrodes 124, 224, 324, and 424 are adjusted and shrunk todesirable values, which results in the advantageous reduction of theCritical Dimension (CD, the gate width) of the resulting transistors,particularly the standard transistors and HV transistors. In accordancewith some embodiments of the present disclosure, the widths of gateelectrodes 124, 224, 324, and 424 may have ratio W112′:W212′:W312′:W412′in the range 1.0:1.0˜1.3:1.3˜1.6:4.0˜4.5,

The subsequently illustrated process steps in FIGS. 5A/5B through 23show the intermediate steps for forming transistors. The process flowfor forming one transistor is illustrated, wherein the process flow mayrepresent the process flow of the standard transistors in device region100, the HV transistors in device region 200, the long-channeltransistors in device region 300, and the IO transistors in deviceregion 400. Accordingly, the components shown in FIG. 5A represent themask stack and the corresponding underlying dummy gate electrode anddummy gate dielectric as shown in FIG. 4, depending on which type oftransistor is to be formed. For example, when a standard transistor isto be formed, the features 22′, 24′, 14, and 16 in FIG. 5A representfeatures 122, 124, 114, and 116, respectively, in FIG. 4. Similarly,when an IO transistor is to be formed, the features 22′, 24′, 14, and 16in FIG. 5A represent features 422, 424, 414, and 416, respectively, inFIG. 4. As shown in FIG. 5A, dummy gate dielectric 22′ and dummy gateelectrode 24′ have sidewall portions (illustrated using dashed lines)extending on the top surface and sidewalls of semiconductor fin 20A. Insubsequent discussion, features 22′, 24′, 14 and 16 are collectivelyreferred to as dummy gate stack 30.

FIG. 5B illustrates a cross-sectional view of the structure shown inFIG. 5A, wherein the cross-sectional view is obtained from the planecrossing line 5B-5B in FIG. 5A. FIG. 5B illustrates that semiconductorfin 20A protrudes higher than the top surfaces of 38A of STI regions 38,and dummy gate stack 30 extends on the top surface and the sidewalls ofsemiconductor fin 20A.

FIG. 6 illustrates the formation of gate spacers 32, which are formed onthe sidewalls of dummy gate stack. The respective step is illustrated asstep 506 in the process flow shown in FIG. 24. In accordance with someembodiments of the present disclosure, gate spacers 32 include aplurality of layers, for example, layer 32A and layer 32B over layer32A. Although not shown, more layers may be included in gate spacers 32.The materials of gate spacers 32 include silicon oxide, silicon nitride,silicon oxynitride, silicon carbo-oxynitride, and/or the like. Forexample, layers 32A and 32B may be formed of the different materials.Alternatively, layers 32A and 32B include same elements (such as siliconand nitrogen) with different compositions (having differentpercentages). Gate spacers 32 may be in contact with the top surfacesand the sidewalls of semiconductor fin 20A in accordance with someembodiments.

Referring to FIG. 7, source/drain regions 36 are formed. The respectivestep is illustrated as step 508 in the process flow shown in FIG. 24. Inaccordance with some embodiments of the present disclosure, theformation of source/drain regions 36 includes etching the portions ofthe semiconductor fin 20A not covered by dummy gate stack 30 and gatespacers 32, and performing an epitaxy to grow epitaxy regions in theresulting recesses. In accordance with some exemplary embodiments, whenan n-type FinFET is to be formed, source/drain regions 36 includesilicon phosphorous (SiP) or phosphorous-doped silicon carbon (SiCP).When a p-type FinFET is to be formed, source/drain regions 36 mayinclude SiGe and a p-type impurity such as boron or indium, which may bein-situ doped during the epitaxy. An implantation may or may not beperformed to dope an n-type (for an n-type FinFET) or a p-type impurity(for a p-type FinFET) into the epitaxy regions. In accordance withalternative embodiments, the formation of source/drain regions 36 isperformed by implanting semiconductor fin 20A.

FIG. 8 illustrates the formation of Contact Etch Stop Layer (CESL) 40and Inter-Layer Dielectric (ILD) 42 over CESL 40. The respective step isillustrated as step 510 in the process flow shown in FIG. 24. Inaccordance with some embodiments, a buffer oxide layer (not shown) isformed on source and drain regions 36 before the formation of CESL 40.The buffer oxide layer may be formed of silicon oxide, and CESL 40 maybe formed of silicon nitride, silicon carbonitride, or the like. Thebuffer oxide layer and CESL 40 may be formed using Atomic LayerDeposition (ALD), for example. ILD 42 may be formed of a flowable oxideusing, for example, Flowable Chemical Vapor Deposition (FCVD). ILD 42may also include Phospho-Silicate glass (PSG), Boro-Silicate Glass(BSG), Boron-Doped Phospho-Silicate Glass (BPSG), Tetra Ethyl OrthoSilicate (TEOS) oxide, or the like. A planarization such as a ChemicalMechanical Polish (CMP) may be performed to level the top surfaces ofdummy gate stack 30, gate spacers 32, CESL 40, and ILD 42.

Dummy gate stack 30 is then removed, as shown in FIGS. 9 and 10. Therespective step is illustrated as step 512 in the process flow shown inFIG. 24. Mask layers 14 and 16 are first removed, and the resultingstructure is shown in FIG. 9. Mask layers 14 and 16 may be removedusing, for example, fluorine-containing process gases such as CF₄/O₂/N₂,NF₃/O₂, SF₆, or SF₆/O₂, or using H₃PO₄ solution. Next, steps are formedto etch an oxide layer, which may be a nature oxide layer, formed ondummy gate electrode 24′. The removal of the oxide may be performedusing NF₃ and NH₃, which react with each other when exposed under RF toform NH₄F, which is used to etch silicon oxide (when heated, for exampleto about 40° C.) to generate (NH₄)₂SiF₆ and water. (NH₄)₂SiF₆ is solid,and when annealed, for example, at a temperature higher than about 100°C., generates gases SiF₄, NH₃, and HF, which are evacuated out ofchamber 44 by a pump (not shown).

FIG. 9 further illustrates the etching of dummy gate electrode 24′ inaccordance with some embodiments of the present disclosure. Wafer 2 isplaced in chamber 44, and process gases NF₃ and H₂ are introduced, andplasma is generated from NF₃, which reacts with H₂ (with plasmagenerated) to form H (hydrogen) radicals and F (Fluorine) radicals. Ionsare also generated from the process gases. FIG. 9 illustrate radicalsrepresented by letter “R,” and positive and negative ions by “+” and “−”signs, respectively. Filter 46 (which may be a selectivity modulationdevice), is used to filter out the ions, while allowing radicals to passthrough filter 46 to reach wafer 2. Fluorine radicals react with silicon(dummy gate electrode 24′) to generate gaseous SiH₄ and H₂, which areevacuated out of chamber 44 by a pump (not shown). During the etching ofdummy gate electrode 24′, fluorine radicals are not directional anddon't have bombardment effect. Accordingly, the bottom corner portionsof dummy gate electrode 24′ are fully etched without leaving residue.

After the removal of dummy gate electrode 24′, dummy gate dielectric 22′is removed, for example, using NF₃/NH₃ or HF, so that the top surfaceand sidewalls of semiconductor fin 20A are exposed, and recess 48 isformed, as shown in FIG. 10. Advantageously, no dummy gate dielectric isleft directly underlying gate spacers 32 due to the etch-through ofdummy gate dielectric layer 22 (FIGS. 2 and 3). Accordingly, no undercutis formed directly under gate spacers 32, wherein the undercut, ifformed, may be filled with the subsequently deposited metal for formingreplacement gate, and may generate a leakage/shorting path. Hence, theetch-through of dummy gate dielectric layer 22 advantageously removesthe leakage/shorting path.

FIGS. 11 and 12 illustrate the formation of gate spacers 50 inaccordance with some embodiments. The respective step is illustrated asstep 514 in the process flow shown in FIG. 24. In accordance withalternative embodiments, the steps shown in FIGS. 11 and 12 are skipped.Referring to FIG. 11, gate spacer layer 49 is formed, for example, usinga conformal deposition method such as ALD or CVD. Accordingly, thethickness of the horizontal portions and vertical portions of gatespacer layer 49 are close to each other, for example, with a differencesmaller than about 20 percent or 10 percent of the thickness of thehorizontal portions. In accordance with some embodiments of the presentdisclosure, gate spacer layer is formed of silicon nitride, siliconcarbo-nitride, silicon oxynitride, or another dielectric material, whichmay be different from the materials of gate spacers 32 and the materialsof CESL 40 and ILD 42. The formation of gate spacers 50 advantageousseparates the subsequently formed metal gate and source/drain regions 36farther away from each other, and reduces the possibility of leakage andelectrical shorting between them.

Referring to FIG. 12, an anisotropic etch is performed to remove thehorizontal portions of gate spacer layer 49, leaving gate spacers 50 onthe sidewalls of gate spacers 32.

Replacement gate stack 52 is then formed in recess 48, and extends onthe top surface and sidewalls of semiconductor fin 20A, as is shown inFIG. 13A. The respective step is illustrated as step 516 in the processflow shown in FIG. 24. Gate stack 52 may include interfacial dielectriclayer 54, high-k gate dielectric 56, and replacement gate electrode 58.In accordance with some embodiments of the present disclosure,interfacial dielectric layer 54 is a silicon oxide layer formed throughthermal oxidation or chemical oxidation, during which a surface layer ofsemiconductor fin 20A is oxidized. The high-k dielectric layer 56 mayhave a k value greater than 7, or greater than 20. Exemplary high-kdielectric materials include hafnium oxide, zirconium oxide, lanthanumoxide, or the like. Replacement gate electrode 58 may be a single-layerformed of a homogeneous conductive material, or may be a composite layerincluding a plurality of layers formed of TiN, Titanium Silicon Nitride(TSN), TaSiN, WN, TiAl, TiAlN, TaC, TaN, aluminum, tungsten, orcombinations thereof. The formation of high-k dielectric layer 56 andgate electrode 58 may include ALD, Physical Vapor Deposition (PVD),Metal-Organic Chemical Vapor Deposition (MOCVD), and/or other applicablemethods. A planarization such as CMP is performed to remove excess gatestack 52.

Hard mask 60 is formed over replacement gate stack 52. In accordancewith some embodiments of the present disclosure, the planarized gatestack 52 is etched back, and hard mask 60 is formed in the resultingrecess left by the etched portion of gate stack 52. The formation ofhard mask 60 includes a deposition step and a planarization step toremove the excess deposited material over gate spacers 32 and ILD 42.Hard mask 60 may be formed of silicon nitride, for example.

FIG. 13B schematically illustrates the profile of the metal gatephysically formed on a wafer. It is observed that replacement gateelectrode 58 may have a profile with a middle portion protruding aboveedge portions. The top surface thus form angle α in the cross-sectionalview as shown in FIG. 13B. Replacement gate electrode 58 may have acurved top surface, with a middle portion of the curved top surfaceprotruding above edge portions. In an embodiment, high-k gate dielectric56 may have a continuously curved or sloping top surface. In anembodiment, high-k gate dielectric 56 may have a convex top surface thatcurves continuously from a first sidewall of the high-k gate dielectric56 to a second sidewall of the high-k gate dielectric 56. High-k gatedielectric 56 may directly contact gate spacers 50 at higher points thanpoints at which high-k gate dielectric 56 contacts replacement gateelectrode 58. As discussed in preceding paragraphs, the illustratedtransistor may represent a standard transistor, an HV transistor, along-channel transistor, and an IO transistor. Due to different widthsof the gate electrode, the profile of these transistors may be differentfrom each other. For example, assuming when the illustrated transistoris a standard transistor, an HV transistor, a long-channel transistor,or an IO transistor, the angle α is equal to α1, α2, α3, or α4,respectively, then ratio α1:α2:α3:α4 may be in the range1.0:1.7˜1.8:1.9˜2.0:2.0˜2.1.

The height H_(MG) of gate stack 52 is measured from the top of hard mask60 to top surface 38A of the STI regions 38 (FIG. 5B), Assuming when theillustrated transistor is a standard transistor, an HV transistor, along-channel transistor, or an IO transistor, the height H_(MG) is equalto height H_(MG1), H_(MG2), H_(MG3), or height H_(MG4), respectively,then ratio H_(MG1):H_(MG2):H_(MG2):H_(MG3):H_(MG4) may be in the range1.0:1.0˜1.1:0.91.0:1.0˜1.1.

The volume V of gate stack 52 may be represented as V1, V2, V3, or V4when the illustrated transistor is a standard transistor, an HVtransistor, a long-channel transistor, or an IO transistor,respectively. In accordance with some embodiments of the presentdisclosure, ratio V1:V2:V3:V4 may be in the range1.0:0.9˜0.98:30˜38:42˜48.

The tilt angle β1 of the sidewall of gate spacers 55 may be smaller thanabout 89 degrees, and tilt angle β2 of the sidewall of gate spacers 32may also be smaller than about 89 degrees. In accordance with someembodiments of the present disclosure, the edges of gate stack 52 aresubstantially straight, and hence tilt angles β1 and β2 are close to 90degrees.

FIGS. 14 through 16 illustrate the formation of lower source/draincontact plugs. The respective step is illustrated as steps 518 in theprocess flow shown in FIG. 24. Referring to FIG. 14, sacrificialdielectric layer 62 is formed, followed by the application of patternedphoto resist 64. Sacrificial dielectric layer 62 is formed of adielectric material different from the material of ILD 42. For example,sacrificial dielectric layer 62 may be formed of a dielectric materialselected from the same candidate dielectric materials for forming ILD42, while they are still formed of different materials. Next, as shownin FIG. 15, sacrificial dielectric layer 62, ILD 42, and CESL 40 areetched to form contact openings 66. Source/drain silicide regions 68 arethen formed, for example, through a self-aligned silicidation process.It is appreciated that source/drain contact openings 66 may be formed ina single lithography process, or may be formed in a double patterningprocess using two lithographs processes, wherein the pattern of thesource/drain contact opening 66 on the left side of replacement gatestack 52 is in a first lithography mask, and the pattern of thesource/drain contact opening 66 on the right side of replacement gatestack 52 is in a second lithography mask. Photo resist 64 is thenremoved.

Referring to FIG. 16, contact openings 66 are filled with a conductivematerial(s), followed by a planarization process, hence formingsource/drain contact plugs 70. In accordance with some embodiments,source/drain contact plugs 70 include a conductive barrier layer formedof titanium, titanium nitride, tantalum, or tantalum nitride, and ametal such as tungsten, aluminum, copper, or the like over the diffusionbarrier layer. In accordance with alternative embodiments, contact plugs70 are formed of a single layer formed of a homogenous material such astungsten or an alloy.

FIGS. 17 through 19 illustrate the formation of a gate contact plug. Therespective step is illustrated as step 520 in the process flow shown inFIG. 24. Referring to FIG. 17, a photo lithography process is performedusing a lithography mask (not shown) to etch-through sacrificialdielectric layer 62. Hard mask 60 is then removed, forming opening 72.In accordance with some embodiments of the present disclosure, theformation of opening 72 includes an anisotropic etching to etch throughsacrificial dielectric layer 62, and an isotropic etching (dry or wet)to remove hard mask 60. The sidewalls of gate spacers 50 are thusexposed. In the embodiments in which gate spacers 50 are not formed, thesidewalls of gate spacers 32 are exposed to opening 72. The etchant foretching sacrificial dielectric layer 62 and hard mask 60 are selected,so that gate spacers 50 and 32 are substantially not etched. Inaccordance with alternative embodiments of the present disclosure,opening 72 is narrower than hard mask 60, and hence some edge portionsof hard mask 60 are left, wherein the corresponding opening 72 and hardmask 60 are illustrated using dashed lines 72′.

Referring to FIG. 18, a conductive material 74 is deposited, wherein asingle layer or a composite layer (including a plurality of conductivelayers) is formed. The material and the structure of gate contact plug74 may be selected from the same candidate materials and structures ofcontact plugs 70. Next, a planarization step such as CMP is performed toremove sacrificial dielectric layer 62 and the portions of theconductive material 70 inside and over sacrificial dielectric layer 62.Accordingly, gate contact plug 74, is formed, as shown in FIG. 19.Source/drain contact plugs 70 are also lowered.

Gate contact plug 74′ and source/drain contact plugs 70 have topsurfaces substantially coplanar with the top surfaces of gate spacers 32and ILD 42. Furthermore, gate spacers 50 (or 32 if gate spacers 50 arenot formed) extend to contact the sidewalls of gate contact plug 74′.Alternatively stated, the sidewalls of gate contact plug 74′ and thesidewalls of gate stack 52 are contact with the same sidewalls of therespective gate spacers 50 (or 32) in accordance with some embodiments.Gate spacers 50 and 32 thus separate gate contact plug 74′ fromsource/drain contact plugs 70. The addition of gate spacers 50advantageously reduces the possibility of the leakage or the electricalshorting between gate contact plug 74′ and source/drain contact plugs70.

In accordance with alternative embodiments in which hard mask 60 are notfully removed, gate contact plug 74′ is separated from one or both gatespacers 32 by remaining portion(s) of hard mask 60, wherein the topsurface of hard mask 60 is also coplanar with the top surfaces of gatespacers 32 and ILD 42. The dashed lines illustrate the sidewalls of gatecontact plug 74′ in accordance with these embodiments.

FIG. 20 illustrates the formation of etch stop layer 76, ILD 78, andsource/drain contact plugs 82 in etch stop layer 76 and ILD 78. Etchstop layer 76 may comprise silicon carbide, silicon oxynitride, siliconcarbo-nitride, or the like. ILD 78 may include a material selected fromPSG, BSG, BPSG, Fluorine-doped Silicon Glass (FSG), TEOS, or othernon-porous low-k dielectric materials. Etch stop layer 76 may be formedusing a deposition method such as CVD. ILD 78 may be formed using spincoating, Flowable Chemical Vapor Deposition (FCVD), or the like, orformed using a deposition method such as Plasma Enhanced Chemical VaporDeposition (PECVD), Low Pressure Chemical Vapor Deposition (LPCVD), orthe like.

ILD 78 and etch stop layer 76 are etched to form openings (occupied by82 and 80). The etching may be performed using, for example, ReactiveIon Etch (RIE). In a subsequent step, contact spacers 80 are formed.Contact spacers 80 may be formed of a dielectric material selected fromSiN, SiON, SiCN, SiOCN, AlON, AlN, combinations thereof, and/ormulti-layers thereof. The formation method may be essentially the sameas the formation of gate spacers 50, which includes a blanket depositionand an anisotropic etching. Contact plugs 82 are then formed, forexample, through a deposition and a planarization. The respective stepis illustrated as step 522 in the process flow shown in FIG. 24.

FIG. 21 illustrates the formation of etch stop layer 84, dielectriclayer 86, conductive vias 88, and via openings 90. Etch stop layer 84and dielectric layer 86 may be formed using similar (or different)materials and similar methods as etch stop layer 76 and ILD 78,respectively. Vias 88 may include a barrier layer formed of titanium,titanium nitride, tantalum, or tantalum nitride, and a conductivematerial such as copper, tungsten, or the like over the barrier layer.Opening 90 is formed by etching layers 76, 78, 84 and 86.

Next, opening 90 is filled to form via 92, as shown in FIG. 22, whichmay be formed of similar material as vias 88. The respective step isillustrated as step 524 in the process flow shown in FIG. 24. Viaspacers 94 may be formed on the sidewalls of vias 88 and/or 90 forreducing leakage or electrical short. FIG. 23 illustrates the formationof bottom metallization layer, which includes metal lines 96.

The embodiments of the present application have some advantageousfeatures. Through the trimming of mask stacks, the widths of sometransistors are reduced, and the sizes of the corresponding transistorsare reduced. The etch-through of dummy gate dielectric reduces thepossibility of electrical shorting and leakage caused by the undercutformed under gate spacers. The formation of additional gate spacers inthe recess left by dummy gate stack advantageously reduces theelectrical shorting and leakage also. The etching of dummy gateelectrode using radicals results in a better removal without leavingresidue. Furthermore, the formation of contact spacers also reduces theleakage and the possibility of shorting between gate contact plug andsource/drain contact plugs.

In accordance with some embodiments of the present disclosure, a methodincludes forming a dummy gate stack over a semiconductor region, forminga gate spacer on a sidewall of the dummy gate stack, removing the dummygate stack to form an opening, forming a replacement gate stack in theopening, recessing the replacement gate stack to form a recess, fillingthe recess with a conductive material, and performing a planarization toremove excess portions of the conductive material over the gate spacer.A remaining portion of the conductive material forms a gate contactplug. A top portion of the gate contact plug is at a same level as a topportion of the first gate spacer.

In accordance with some embodiments of the present disclosure, a methodincludes forming a dummy gate stack on a top surface and a sidewall of asemiconductor fin, forming gate spacers having sidewalls contactingsidewalls of the dummy gate stack, forming a source/drain region on aside of the dummy gate stack, forming an inter-layer dielectric to coverthe source/drain region, removing the dummy gate stack to form anopening between the gate spacers, filling a bottom portion of theopening with a replacement gate stack, and forming a gate contact plugto fill a top portion of the opening. The gate contact plug is betweentop portions of the gate spacers.

In accordance with some embodiments of the present disclosure, a deviceincludes a semiconductor region, a gate stack over the semiconductorregion, a source/drain region on a side of the gate stack, and a firstgate spacer and a second gate spacer on sidewalls of the gate stack. Agate contact plug is over the gate stack, and the gate contact plug isbetween the first gate spacer and the second gate spacer, with topportions of the first gate spacer and the second gate spacer being at asame level as the gate contact plug.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A device comprising: a semiconductor substrate;isolation regions extending into the semiconductor substrate; asemiconductor region protruding higher than top surfaces of theisolation regions; a gate stack on a sidewall and a top surface of thesemiconductor region, wherein an entirety of a topmost surface of thegate stack is curved, wherein the gate stack comprises: an interfacialdielectric layer; a gate dielectric; and a gate electrode over the gatedielectric, the gate electrode having a curved top surface, wherein acenter portion of the top surface of the gate electrode protrudes aboveedge portions of the curved top surface of the gate electrode; asource/drain region on a side of the gate stack; a first gate spacer anda second gate spacer on opposite sidewalls of the gate stack, whereinthe first gate spacer and the second gate spacer contact the gate stack,wherein the gate dielectric contacts the first gate spacer at a firstpoint higher than a second point where the gate dielectric contacts thegate electrode, wherein the gate dielectric has a first sidewall thatcontacts the first gate spacer and a second sidewall that contacts thegate electrode, wherein a topmost point of the first sidewall is higherthan a topmost point of the second sidewall, wherein the topmost pointof the first sidewall is the highest point at which the gate dielectricdirectly contacts the first gate spacer, wherein the curved top surfaceof the gate electrode contacts the topmost point of the second sidewall,wherein the center portion of the curved top surface of the gateelectrode is higher than the topmost point of the first sidewall and thetopmost point of the second sidewall; a third gate spacer and a fourthgate spacer, with the first and the second gate spacers between thethird gate spacer and the fourth gate spacer, wherein the third gatespacer contacts the first gate spacer, and wherein the first gatespacer, the second gate spacer, the third gate spacer and the fourthgate spacer physically contact the top surface of the semiconductorregion; and a fifth gate spacer and a sixth gate spacer, with the thirdand the fourth gate spacers between the fifth gate spacer and the sixthgate spacer, wherein the fifth gate spacer contacts the third gatespacer, wherein the first gate spacer, third gate spacer, and fifth gatespacer comprise materials that are different from each other, wherein afirst interface between the interfacial dielectric layer and the gatedielectric is higher than a second interface between the third gatespacer and the fifth gate spacer.
 2. The device of claim 1 furthercomprising a gate contact plug over the gate stack, wherein the gatecontact plug is between the first gate spacer and the second gatespacer, with top portions of the first gate spacer and the second gatespacer being at a same level as the gate contact plug, wherein the gatecontact plug is in contact with sidewalls of the first gate spacer andthe second gate spacer.
 3. The device of claim 1, wherein top surfacesof the first gate spacer, the third gate spacer and the fifth gatespacer are at a same level.
 4. The device of claim 1 further comprising:a first source/drain contact plug over and electrically coupled to thesource/drain region, wherein the first source/drain contact plugcomprises a top surface substantially level with top surfaces of thefirst gate spacer and the second gate spacer; and an etch stop layerover the first source/drain contact plug and the gate contact plug. 5.The device of claim 4 further comprising: a dielectric layer over theetch stop layer; a second source/drain contact plug over and contactingthe first source/drain contact plug, wherein the second source/draincontact plug is in the etch stop layer and the dielectric layer; and adielectric contact spacer encircling and contacting the secondsource/drain contact plug.
 6. The device of claim 1, wherein the thirdgate spacer comprises a first layer having an L-shape.
 7. The device ofclaim 1, wherein a sidewall of the gate contact plug is verticallyaligned to a sidewall of the gate stack in a cross-sectional view of thedevice.
 8. A device comprising: a semiconductor substrate; isolationregions extending into the semiconductor substrate; a semiconductor finprotruding higher than top surfaces of the isolation regions; a gatestack on a sidewall and a top surface of the semiconductor fin, whereinan entirety of a topmost surface of the gate stack is curved, whereinthe gate stack comprises: an interfacial dielectric layer; a gatedielectric, wherein the gate dielectric has a first sidewall and asecond sidewall; and a gate electrode, the gate electrode having acurved top surface, wherein a middle portion of the curved top surfaceof the gate electrode protrudes above edge portions of the curved topsurface of the gate electrode, wherein the curved top surface of thegate electrode contacts a topmost point of the first sidewall; a firstgate spacer on a sidewall of the gate stack, wherein the first gatespacer contacts the second sidewall, wherein a topmost point of thesecond sidewall is higher than the topmost point of the first sidewall,wherein the gate dielectric has a curved top surface that curvescontinuously from the topmost point of the first sidewall to the topmostpoint of the second sidewall; a second gate spacer between and incontact with the first gate spacer and a third gate spacer, wherein thefirst gate spacer and the second gate spacer physically contact the topsurface of the semiconductor fin, wherein the second gate spacer and thethird gate spacer each comprise a first element, wherein a percentage ofthe first element in the second spacer is different than a percentage ofthe first element in the third spacer, wherein a first interface betweenthe interfacial dielectric layer and the gate dielectric is higher thana second interface between the second gate spacer and the third gatespacer; a dielectric hard mask overlapping the gate dielectric; and adielectric layer over and in contact with top surfaces of the first gatespacer, the second gate spacer, the third gate spacer, the dielectrichard mask, and a first gate contact plug.
 9. The device of claim 8further comprising the first gate contact plug over and contacting thegate electrode, wherein a top surface of the first gate contact plug islevel with a top surface of the first gate spacer, the second gatespacer, and the third gate spacer.
 10. The device of claim 8, whereinthe dielectric layer is an etch stop layer, and the device furthercomprises: a first inter-layer dielectric over the etch stop layer; asecond inter-layer dielectric over the first inter-layer dielectric; afirst source/drain contact plug in the first inter-layer dielectric; asecond source/drain contact plug in the second inter-layer dielectric,wherein the second source/drain contact plug is over and contacting thefirst source/drain contact plug; and a second gate contact plugpenetrating through both the first inter-layer dielectric and the secondinter-layer dielectric.
 11. The device of claim 10 further comprising adielectric contact spacer encircling the second gate contact plug,wherein the dielectric contact spacer penetrates through both the firstinter-layer dielectric and the second inter-layer dielectric.
 12. Thedevice of claim 10 further comprising a dielectric contact spacerencircling the first source/drain contact plug.
 13. The device of claim10 further comprising a dielectric contact spacer encircling the secondsource/drain contact plug.
 14. The device of claim 8, wherein the firstgate contact plug comprises a side edge in contact with a side edge ofthe dielectric hard mask.
 15. The device of claim 8, wherein a bottomsurface of the interfacial dielectric layer is in contact with thesemiconductor fin and the gate dielectric has sidewall portions higherthan, and connected to opposite ends of the interfacial dielectriclayer.
 16. The device of claim 8 further comprising an additionalinter-layer dielectric, with the gate stack and the first gate spacer inthe additional inter-layer dielectric, wherein a top surface of theadditional inter-layer dielectric is further in contact with thedielectric layer.
 17. A device comprising: a semiconductor substrate;isolation regions extending into the semiconductor substrate; asemiconductor fin protruding higher than top surfaces of the isolationregions; a gate stack on a sidewall and a top surface of thesemiconductor fin, wherein a topmost surface of the gate stack comprisesmultiple curves, wherein the gate stack comprises: an interfacialdielectric layer; a gate dielectric, wherein the gate dielectric has atop surface that is curved, wherein the gate dielectric has a firstsidewall and a second sidewall, the curved top surface of the gatedielectric being convex and curves continuously from the first sidewallto the second sidewall; and a gate electrode, a middle portion of a topsurface of the gate electrode protruding above edge portions of the topsurface of the gate electrode, wherein the top surface of the gateelectrode is curved, wherein the gate electrode contacts the firstsidewall; a first gate spacer comprising a first portion and a secondportion contacting opposite sidewalls of the gate stack, wherein thesecond portion of the first gate spacer contacts the second sidewall,wherein a topmost point of the second sidewall is higher than a topmostpoint of the first sidewall and lower than a topmost point of the middleportion of the top surface of the gate electrode; a second gate spacercomprising a first portion and a second portion contacting the firstportion and the second portion of the first gate spacer respectively,wherein the first portion and the second portion of the first gatespacer and the first portion and the second portion of the second gatespacer physically contact the top surface of the semiconductor fin; anda third gate spacer comprising a first portion and a second portioncontacting the first portion and the second portion of the second gatespacer respectively, wherein the first gate spacer, the second gatespacer, and the third gate spacer comprise materials that are differentfrom each other, wherein a first interface between the interfacialdielectric layer and the gate dielectric is higher than a secondinterface between the first portion of the second gate spacer and thefirst portion of the third gate spacer.
 18. The device of claim 17further comprising: a first gate contact plug over the gate stack, witha top surface of the first gate contact plug being substantiallycoplanar with a top surface of the first gate spacer, wherein oppositeside edges of the first gate contact plug contact side edges of thefirst portion and the second portion of the first gate spacer; a secondgate contact plug over and contacting the first gate contact plug; and acontact spacer comprising portions contacting opposite sidewalls of thesecond gate contact plug.
 19. The device of claim 18, wherein a bottomsurface of the contact spacer contacts a top surface of the first gatecontact plug.
 20. The device of claim 17 further comprising an etch stoplayer, with a bottom surface of the etch stop layer contacting both thetop surface of the first gate contact plug and the top surface of thefirst gate spacer.